[result][mongodb] Fixes as_uri() when not configured by url (Issue #3094)

celery/backends/mongodb.py

@@ -252,6 +252,8 @@ class MongoBackend(BaseBackend):
         :keyword include_password: Censor passwords.
 
         """
+        if not self.url:
+            return 'mongodb://'
         if include_password:
             return self.url